Various electronic systems are capable of processing digital audio signals and amplifying the processed signals to drive a speaker, thereby producing sound waves. Examples of such systems include portable media player devices, cellular telephones, smartphones, tablets, computers, radios, audio recorders, stereo equipment (e.g., audio receivers), components in a vehicle, and the like. For digital audio processing, an encoder-decoder (CODEC) may be used to convert analog audio signals to encoded digital signals and vice versa. For example, a CODEC may receive an analog audio signal (e.g., from a microphone), and convert the analog audio signal into a digital signal that can be processed (e.g., digitally filtered) via a digital signal processor (DSP). The CODEC can then convert the processed digital output of the DSP to an analog signal for use by audio speakers, for example, via a digital-to-analog converter (DAC).
Amplification of digital or analog audio signals may be performed using any of various suitable techniques. Class-D amplifiers are widely used in audio applications because these types of amplifiers may be more efficient than class-AB amplifiers and involve less heat management and external components (e.g., heatsinks). A class-D amplifier generally refers to an electronic amplifier in which the transistors in the output stage operate as electronic switches, instead of as linear gain devices, as in other amplifier types. In a class-D amplifier, the signal to be amplified is a train of pulses of constant amplitude, but varying width and separation (e.g., different duty cycle), so the output stage transistors switch rapidly back and forth between fully conductive and nonconductive states. Before being applied to the amplifier, the signal to be amplified is converted to a pulse train using pulse width modulation, pulse density modulation, or other suitable techniques. The amplified pulse train output by the transistors can be converted back to an analog audio signal by low-pass filtering the pulse train to remove the unwanted high-frequency components introduced by pulse modulation and recover the desired low-frequency signal.
Despite their benefits, class-D amplifiers may have some drawbacks, such as lower linearity and/or lower power supply rejection ratio (PSRR) in certain aspects when compared to other amplifier implementations. In order to improve the overall performance of class-D amplifiers, feedback can be applied around the output power stage. This feedback may increase the linearity of the class-D output stage and may attenuate power supply ripple present in the audio band (e.g., intermodulation products between the main signal and the power supply tones).